Koeltorenboekje Engels Zonder Logo

7/29/2019 Koeltorenboekje Engels Zonder Logo

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Section page

1. Cooling tower operation 2

2. Cooling tower types 4

3. Cooling tow er notions 5

4. Heat exchange 6

5. Dynamic cooling tower functioning 9

6. Water consumption 14

7. Cooling tower regulation 16

8. Measuring the cooling tower capacity 18

9. Cooling tower components 23

10. Sound of the cooling tower 27

Version 1

Cooling tow ers 1

Content


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1.0 Why cooling towers?Cooling towers are, amongst oth ers, used foran efficient cooling of cooling w ater in theprocess and food indu stry and for thecooling of w ater cooled cooling m achines infor example air conditioning installations.Since the use of surface and spring water is

less and less desirable, the practice of watercooling by cooling towers is increased.Before, water often w as cooled with the h elpof cooling and sprayin g pon ds. Later, thesewere substituted by the w ell-known naturaldraught cooling towers for large waterdebits. The principle of operation of thesecooling tow ers is based on evapor ation of asmall amou nt of the circulating coolingwater. Therefore, these towers are also calledevaporative cooling towers. Because of theavailability of electricity an d the d esire to

build in a more comp act way, the forceddrau ght cooling towers became increasinglypop ular. This type of cooling tow er isprovided w ith one or more fans.

1.1 Wet bulb temperatureThe wet bulb temperature is an essentialnotion for th e selection of an evap orativecooling tow er. This air temperatu re caneasily be measured with th e help of apsychrom eter. Usually, this is a g lass thermo-meter filled with mercury and it is put in acotton cover. This cover is soaked in distilledwater and w hen un saturated air passes,water from the cover evap orates (figure 1).For this evapo ration (latent) heat is extractedfrom the cover, causing it to cool dow n. Nowthe temp erature of the cotton cover is lowerthan that of the p assing air. Subsequ ently,(sensible) heat flows from the air to thecover. Influen ced by th e passing air, thecover gradually takes a temperature suchthat th e heat flow of the air to the cover isexactly the same as th e heat that is required

for the evaporation of the water from the wetcover. This balanced temp erature of th e air iscalled the w et bulb temp erature (Tnb).

Put d ifferently, for the wet bu lb temperatu reof the wet cover the passing u nsaturated airemits just enough sensible heat to supportthe latent heat flow (evaporation) of thewater. The wet bu lb temperatu re is also

called the adiabatic saturation temperature.

Cooling towers 1. Cooling tower operation 2

1. Cooling tower operation

Thermometer

Air flow

Wet cotton cover

figure 1. Psychrometer principle

Cooling tower make Polacel


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1.2 Principle of operationIn an evap orative cooling tow er circulatingcooling wa ter that is brough t in directcontact with the d rawn-in surround ing air iscooled. A process warm cooling w ater isspread out th inly over a contact body, wh ichpreferably has a large exchan ge sur face. Theair is led equ ally over this film of water.

The cooling w ater is cooled by evaporationof a small amount (1-2%) of the circulatingwater. Per kg evap orated cooling water

2491 kJ is extracted from the circulatingcooling w ater. The un saturated air flowabsorbs the evaporated water and the heatbelonging to it. The air stream is furtherheated in the contact bod y (cooling fill) unt ila heat-exchan ge balance between the airflow an d t he circulating cooling water isreached.The decrease of the temp erature of thecooling w ater and the increase of the air sheat content in the cooling tower is a gradualprocess.

The operation of an (evapor ative) coolingtower is based on t he pr inciple of combinedheat an d mass tran sfer. This is a transferof sensible (dr y) heat betw een circulatingcooling wa ter and passing air by convectionand the transfer of latent (wet) heat byevaporat ion of the cooling w ater. Thiscombined h eat and m ass transfer in acounter flow cooling tower is clarified inthe Mollier diagram for wet air (figure 2):Air with condition A and a matching wetbulb temperature Tnb1 passes a wet surfacewith th e temperatu re Tw2. The condition of the saturated air in the border between airand water (just abov e the wet sur face) equalsthe p oint Tw2, lying on th e saturation line inthe Mollier diagram.When air and water pass one another incounter flow, the colder air comes intocontact with the warm cooling water and

the air absorbs more heat.For this reason the cond ition of the airmoves to the right and follows the curveA-B. In th e point B, with a matching wet

bulb tem peratu re Tnb2, is a curv e directedto the p oint Tw1. The air flow absorbs m oreand more water and will eventually be satu-rated. The force behind th e total enthalpyincrease of the air is the en thalpy differencebetween th e air in the border layer air-waterand the passing air.

In short, in th e cooling tow er the coolingwater is cooled from temp erature Tw1 toTw2. The drawn -in air with condition Tnb1is heated and hum idified to a conditionTnb2, wh ere the air is almost saturated .A rule of thum b is that the wet bulb tempe-ratu re of the air leaving the cooling tow eralmost equ als the average of the coolingwaters in and outlet temperatures. Thisdep end s on the cooling tow ers efficiency.

Cooling towers 1. Cooling tower operation 3

Tw1

Water

A

BAir

Tnb2

Tw2

Tnb1

MOLLIER-CHART

figure 2. Principle of a evaporative cooling tower


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2.0 Natural draughtcooling tower

From early days on the natural d raught cool-ing tower is the m ost well-know n of coolingtowers. The main characteristic of this typ eof cooling tow er is its parabolic shape.Usually prod uced in a bod y of reinforced

concrete, there are pieces with a d iameter of no fewer than 40 meters and a buildingheight of 100 meters. The m ost commonapp lication is for cooling of very large waterdebits (figure 3). Because the warm coolingwater is equally distributed, a naturaldraught of the air flow in counter flow withthe water occurs in the body. This effect isalso called the chimney effect because of the fact that hot air rises. Because of this airflow evap oration an d cooling of the circula-ting cooling water occurs.

2.1 Forced draughtcooling tower

The characteristic of forced d raug ht coolingtowers is that one or m ore fans cause an airflow in th e cooling tower. This causes a sig-nificantly high er air speed in the coolingtower and a better cooling than in the n atur-al draft cooling tow er per m 2 surface. Thedisadvantage of the required energy for thefan is outbalanced by the advantage of buil-ding for lower costs because the mann er of building is more compact.

The most well known examples of thiscategory of cooling towers are th e counterflow and the cross flow cooling towers. In acounter flow cooling tower th e water fallsdow n in a vertical manner and the air risesin op posite d irection. In a cross flow coolingtower th e water falls in a v ertical mann er,crossed by th e air flow in horizon taldirection.

Cooling towers 2. Cooling tower types 4

2. Cooling tower types

Hot air

Warm water

Cold air

Cold water

figure 3. Natural draft cooling tower


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When selecting a cooling tow er, the coolingwater temperatures as well as the wet bulbtemperatures are important data.The cooling water inlet temp erature (Tw1) isthe temp erature of the circulating coolingwater d ebit (Qw) that is led into the coolingtower. The cooling w ater outlet temp erature

(Tw2) is the temp erature of the cooling w aterleaving the cooling tow er.The difference between th e in and outlettemp eratures of the cooling w ater is alsocalled range (Tw1-Tw2).The difference between the ou tlet tempera-ture of the cooling water and the wet bulbtemp erature is also called approach(Tw2-Tnb).

The cooling capacity (KC) of a cooling toweris calculated as follows:

KC = (Tw1 - Tw2) * 4,187 * 1000 * Qw(kW) (C) (kJ/ kg) (kg/ m 3) (m 3 / s)

specific heat of water = 4,187 kJ/ kg specific mass of water = 1000 kg/ m 3

The cooling capacity of a cooling tower isalways equ al to the cooling load. This isthe energy that is added to the circulatingcooling wa ter du ring the cooling process.The efficiency ( ) of a cooling tow er in aformula is:

= Tw1 - Tw2Tw1 - Tnb

The theoretically lowest possible coolingwater ou tlet temperatu re (Tw2) in anevapor ative cooling tower is equal to the w etbulb tem peratu re (Tnb). In that case theefficiency of the cooling tow er is 100%. Forthis to be realised the cooling tower shou ldbe infinitively large. For this reason we can

say that the water outlet temperature cannever be lower than the wet bulb temp eratu-re of the draw n-in air in the cooling tow er.

Another frequently used notion in thecooling tow er technique is the water load(R), also called Q over A ratio. This is theamou nt of circulating cooling w ater persquare m eter wet cooling tow er surface(m 3 / m 2 / h).

Cooling towers 3. Cooling tower notions 5

3. Cooling tower notions


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4.1 Cooling fillsThe most important p art of the heat andmass exchange between water and air takesplace in the cooling fill of the cooling tower.As a contact body the cooling fill exists inman y constructions. We have wav edsynt hetic foils (film fills), which exist of

manageable blocks and the well-knownwooden splash bars. When the w ater isslightly polluted, nowadays synthetic splashfills are used as well.

In order to achieve a maximum heat exchan-ge in th e cooling fill, the following p ointsmust be considered: The cooling water should cover a maxi

mally large surface with respect to the airstream. Therefore, a thin w ater film an d alarge exchange surface (m 2 surface per m 3

contents) of the cooling fill are of greatimportance. The speed of the heat exchange is enlarged

by a m ore turbulent air stream (amongothers a high air speed).

The rougher the surface of the cooling fill,the more turbulent is the air/ water stream.

A longer stay or contact period of thecooling w ater in th e cooling fill enhancesthe heat exchange between water and air.Usually, the stay is stretched by enlargingthe depth / height of the cooling fill (in thedirection of the air).

A good water distribution above the

cooling fill du e to a correctly d esignedwater distribution system in the coolingtower.

An equal distribution of air speed in thecooling fill du e to an aerod ynam ic coolingtower.

Film fillIn the m odern synth etic film fill (figure 4b)the wa ter is distributed ov er a large surfacein a th in, levelled film of water. For th is typeof cooling fill it is necessary to have a good

water d istribution system. Film fills areavailable for coun ter flow as w ell as forcross flow cooling tow ers.Due to th e crossed chann el structure of thefilm cooling fill, which u sually has a meshwidth of 12 to 19 mm. and a building heightof 600 to 1500 mm ., the air stream is very

Cooling towers 4. Heat exchange 6

4. Heat exchange

figure 4a. Grid fill figure 4b. Film fill

figure 4c. Splashbars


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turbulent. Therefore, the film fill is a veryefficient cooling fill and it is used frequently.The term efficient can be explained in thefollowing way:A large exchang e sur face (ca. 250 m 2 / m 3) ina compact block shape w ith a relatively lowairside resistance. In oth er word s: a lot of cooling capacities per m3 cooling fill againstlow en ergy costs (of the fan).

Bar fillThe dated use of wooden bars in a cooling

tower is based on t he pr inciple of splashing(figure 5). The bars are mou nted in alterna-ting mann er having vertical and horizontaldistances of e.g. 150 mm. betw een them .Cooling fill heights up to 8 m. and depths u pto 3 m. are n o exceptions. The cooling waterdisperses on every bar so that small waterdrop s emerge. The result of this is a moreeffective heat exchange surface. The splasheffect also lengthen s the stay of the coolingwater. In gen eral we can say that thesesplash b ars are a mod erately efficient cooling

fill (a small exchange surface m2

/ m3

).Synthetic splash bars (figure 4c) are a goodalternative for the wooden bars in oldercooling tow ers. Using these, the coolingcapacity and the efficiency of the coolingtower is usu ally enlarged. Splash bars areused more frequently in cross flow coolingtowers than in counter flow cooling towersor than in a combination of these.

Splash fillThe splash fill (figure 4a) is a good alterna-tive of the film fill mentioned above. Themesh width is for example 40 mm. with avertical splash d istance of about 40 mm. Theuse of the splash fill is based on a combina-tion of the sp lash effect of the splash bars

and a thin water film around the splashsur face. The exchan ge su rface is ca. 150 m 2 / m 3. Because of the good red istribution of thecooling water of this cooling fill, there is less

demand on the w ater distribution system inthe cooling tow er. The splash fill is also usedfor extra redistribution of the water and as amechanical protective layer on the film fillswith a h igh nozzle pressure. In counter flowcooling tow ers the overall heights vary from900 to 1500 mm .These splash fills are not very frequentlyused in cross flow cooling towers w ithbigger cooling fill heights.It should be clear th at the splash fill is lessefficient than the film fill, but more efficientthan th e splash bars. The most importantadv antage of th e splash fill over the film fillis that it is less sensitive for p ollution th at isdu e to a lesser quality of the circulatingcooling w ater in the cooling tower.


Air cross flow

Air counter flow

Waterdroplets

figure 5. Splashbars


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4.2 Counter flow vs. CrossflowAs already men tioned in section 2.1, coun terflow an d cross flow cooling tow ers belong tothe category of forced d raug ht coolingtowers. The principle of both these forms of building is sketched in a combined d rawing(figure 6). The counter flow cooling tower isusually produ ced with a w ater distributionsystem w ith p referably full- cone n ozzles.With the cross flow cooling tower a so-called

pressureless water distribution system isusually used, operating by the principles of gravitation. The dispersed water in the coun-ter flow cooling tower n ecessitates the use of a d rift eliminator. This is not alw ays n ecessa-ry for a cross flow cooling tower. For the aircirculation both typ es of cooling towers u seinduced d raught fans that are placed on topof the cooling tow er.

Counter flowThe heat exchan ge in a coun ter flow cooling

tower is schematically d epicted in figure 7.The cooling fill is d ivided in 5 imaginarylayers: Air-sided : In the bottom cooling fill layer

the inlet wet bulb temperature Tnb1increases when ascend ing. The outlet Tnbof the first layer is the inlet Tnb of the second cooling fill layer etc., until thefinal Tnb2 is reached.

Water-sided : In th e fifth cooling fill layerthe inlet water tem peratu re Tw1 d ecreaseswh en descending. The outlet wet Tw of the fifth layer is the inlet Tw of the fourth cooling fill layer etc., un til the fina l Tw2 isreached.

Cross flowFor a cross flow cooling tower (figure 7) theheat exchange can be d epicted in a similarway. Here we d ivide th e cooling fill in 18imaginary b locks: Air-sided : In the blocks 1-3 the inlet wet

bulb temp erature Tnb 1 increases.This is also tru e for th e blocks 4-6, 7-9 etc.The final Tnb p er horizonta l layer

(3 blocks) differs. The values of the outletTnb of the blocks 3-18 is averaged to Tnb2,

the outlet wet bulb temp erature of theentire cooling fill.

Water-sided: When the cooling w ater withan inlet water temperature Tw1 passed theblocks 1-16, 2-17 and 3-18 in v erticaldirection, an average can be taken of thewater ou tlet temperatu re of the blocks 16,17 and 18, the water outlet temperatureTw2 of th e entire coolin g fill.


Air

Water

Water

Counter flow

Drift eliminator

Water distributionsystem

Fill

Cross flow

Air

Drift eliminator

Water distributionreservoir

Fill

4

5

3

2

1

Tnb2 Tw1

Tnb1 Tw2

WaterAir

1 2 3

4 5 6

7 8 9

10 11 12

13 14 15

16 17 18

Tnb2

Tw1

Tnb1

Tw2

Air

Water

COUNTER FLOW CROSS FLOW

figure 6. Principle counter/cross flow

figure 7. Heat transfer principle


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Amplification It may be clear that the air- and water-

sided heat balance should be in equili-brium in every cooling fill layer or block.So the increase in enthalp y of the air mu stbe equal to th e decrease of the heat contentof the cooling water. With th e help of modern calculation techniques the physicalevapor ation process in a coun ter flow orcross flow cooling tow er is thu s calculated.

With the principle of the cross flow wemay notice that th e first block cools the

water the m ost effective, because in h erethe warmest water comes into contact withthe coldest air. Reasoning th is way w e cansay that th e water flowing from b lock 16 iscolder than th e water flowing from block 17 and 18 respectively. This could be areason to m oisten the cross flow fill asym-metrically on top (more cooling water onthe air inlet side of the cooling fill). Theblocks 3-18 do n ot cool the water to th esame degree as the blocks 1-16 and 2-17 (invertical direction). This tells us th at it is n ot

always u seful to select a deep er cooling fill(air sided ). Sometimes, an enlargem ent of for example 50% of the dep th of thecooling fill only raises the cooling capacitywith 15%. In v ertical direction, on the oth erhan d, it is more useful to choose a highercooling fill, on the cond ition that th is is nota problem for the whole construction orthe internal w ater economy.

As a rule the coun ter flow cooling fill isalways m ore effective th an the cross flowcooling fill, because in counter flow lessm 3 cooling fill is required (for example30%) than in cross flow, assum ing th at thedesign data and cooling fill characteristicsas type and m esh width are the same.Depending on the approach the turningpoint of the choice between coun ter flowand cross flow is more closely, as we willexplain in a later article. We then willcompare the advantages and d isadvanta-ges of these two cooling tower typ es inmore depth.

So far, we gave a first theoretical onset inexplaining the fun ctioning of the (evaporati-ve) cooling tower. In this chap ter, the coolingfill is given extra att ention, because thisactually is the heart of the cooling tower. Inthe following chapters we w ill explain thefunctioning of several other cooling tow ercompon ents in more d etail. In th e followingarticle w e will try to clarify severalmisund erstandings concerning theoperation and business operation.



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Norm ally, a cooling t ower is selected o n on edesign cond ition. With this, actually oneoperation p oint of the cooling tower isdetermined, usually the most important one,with a corresponding wet bulb temperature.In order to give an imp ression of the coolingtower p erformance beyond this operation

point, we need a performance curve (figure8). Such a cur ve gives a good ind ication of the cooling tow er functioning in d ifferentclimatologically circumstances, withfluctuating cooling water temperatures andwith a p ossible variation in cooling w aterflow. The air flow throu ghou t the coolingtower is assumed to be the same.Such performance curve immediately showsthat th e cooling tow er is flexibly wor kingequipment. Guided by several questions, wewill explain this dynamic operation:

1.W hich factor determin es the range (delta T)(= the cooling range Tw1-Tw2) of thecirculating cooling water in the cooling tower?

In a cooling tow er this d elta T is alwaysdeterm ined by th e cooling process. Thecooling tow er is adjusted to th is. In oth erwords, the cooling tow er d oes not d eterminethe size of delta T, but takes this from th ecooling process. The cooling tow er followsthe cooling process and not the other wayaround. This was a misund erstanding thatoccurs q uite regu larly in pr actice.

2.W hat is the influence of the approach(Tw2-Tn b) on the size of the required coolingsurface?

By the term cooling su rface we u suallyund erstand the horizontal, wetted su rface of the cooling tower. As already m entioned inthe first article (chapter 3), it is increasinglydifficult for a cooling tower to cool the wa terwith a sm aller approach. The smaller thedifference between the cooling w ater ou tlettemperature and the w et bulb temperature,

the bigger the d imensions of the coolingtower sh ould be (see figure 9). For this thewater load (m 3 / m 2 / h), th e coolin g r an ge, th e

air speed and the wet bulb temperature areassum ed to b e constant. Obviously, this is

also the case with an u nvar ying choice of the cooling tower an d t he cooling fill.

Cooling towers 5. Dynamic cooling tower functioning 10

5. Dynamic cooling tower functioning

Tnb ( C)18,0 20,0 22,0

26,2

25,0

23,8

T = 10

T = 7

T = 4

Tw2 ( C)

18

16

14

12

10

8

6

4

2

2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

APPROACH (C)

REQUIRED SURFACE (M 2)

figure 8. Performance curve

figure 9. Example required wet surface


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Example:approach [C]: 8 4

[%]: (100) (50)

cooling tower [m 2]: 5 10surface [%]: (100) (200)

From the above example we can concludethat when the ap proach is divided into two,the required cooling tower surface must betwice as big. This shows u s that everyincrease or d ecrease in d egrees Celsius of

the ap proach is of direct influence on th erequired cooling tower su rface. One needsto be aware of this w hen selecting a coolingtower. When a high er selection w ater outlettemp erature of the cooling tow er is notacceptable, maybe a slight d ecrease of thewet bu lb temperatu re in the selection is (seequestion 6). An economical guid e value w iththe selection of a cooling tow er is anapp roach of 4 to 5 C. With th is, we yet againpoint to th e fact that an app roach increase of 4 C with 1 C to 5 C results in a decrease of

the cooling tower surface of 20%.

3. What hap pen s to the app roach (Tw2-Tnb)when th e wet bulb temperature changes?The cooling ran ge (delta T) is taken as a con-stant value. When th e wet bulb temp eratureincreases, the cooling w ater temp erature andthe ou tlet temperatu re will increase as well.The entire cooling range Tw1-Tw2 thenchanges to a higher temperature. A decreaseof the wet bulb temp erature, on the otherhand , results in a decrease of the entirecooling ran ge. Herewith it is incorrect tosupp ose that when the wet bulb temperatureincreases with 1 C, the cooling ran ge alsoincreases with 1 C. As a ru le of thumb wecan assume that w hen the w et bulb tempera-ture increases with 1 C the cooling rang edoes so w ith 0,6 C. The cooling wat er out lettemperature then is 0,6 C higher and theapproach is 0,4 C lower.

Example:Tw1 [C]: 30,8 32,0 33,2Cooling range [C]: 7,0 7,0 7,0Tw2 [C]: 23,8 25,0 26,2

Approach [C]: 5,8 5,0 4,2Tnb [C]: 18,0 20,0 22,0

efficiency [-]: 0,55 0,58 0,63

In spring, autum n and w inter the outsidetemperature, and therefore the wet bulbtemperature, usually is lower than in su m-mer. This means th at the cooling tower canreach the required outlet temperature mostof the year, because then th e app roach isbigger. Put differently, the cooling towersdim ensions are actually too large for themost of the year, un less the user w ants toreach a colder w ater temperature than thetemp erature for wh ich the tow er is selected.

In the above example it is apparent that thecooling tower work s less efficient w ith alower wet bulb temperature and somewhatmore efficient with a h igher wet bu lb tempe-rature. With this we assume standard water/ air debit proportions in the cooling tower.


28.5

26.5

22.5

22.0

R = 13 R = 8

Tw2

Time

Tw1

T = 4T = 6.5

Temp.( C)

figure 10. Variable waterflow


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4. W hat is the influence of a variation in thecooling water debit (with equal cooling load) onthe cooling water temperatu res?

As a starting point we take the examplegiven in figure 10. In the starting situation aconstant w ater load (R) of 13 m 3 / m 2 / hr onthe cooling tower, a constant wet bu lbtemperature and a water inlet temperatureof 22,5 C with a correspond ing cooling loadvalue of (13*4=) 52 are assumed. When thewater load in the cooling tower is suddenlydecreased to 8 m 3 / m 2 / hr, th e d elta T will

increase from 4 to 6,5 C. The cooling loa dremains constant with a value of (8* 6,5=) 52.It should be noted that the w ater outlet tem-peratu re Tw2 is d ecreased w ith 0,5 C. In thisexample it is thus possible to obtain some-what colder water from the cooling tower bylessening th e water d ebit over the coolingtower. It must be remarked that thecorrespon din g increase of the w ater inlettemp erature in the cooling process can beunacceptable. Furthermore, the water distri-bution system is in many cases not designed

for great variation in the cooling wa ter flow.

5. Is a cooling tower able to cool a larger coolingload than it was originally designed for?

The answer to this qu estion is yes. This isillustrated by figure 8. In ou r examp le aconstant w et bulb tem peratu re of 20 C ischosen. With a delta T of 7 C, the wateroutlet temp erature Tw2 of the cooling tow eris 25,0 C. When the cooling load of thecooling process increases to a delta T of 10 C, the wat er outlet temp erature of thecooling tower increases to about 26,7 C. Anincrease of the cooling load of 43% results inan in crease of the Tw2 of the cooling tow erof 1,7 C. In t his examp le the ap proachincreases from 5 C with 1,7 C to 6,7 C;this is an increase of 34%.

In practice we sometimes hear a rem ark of the kind : the cooling tower only cools forhalf its capacity. The cooling tower then isdesigned for a capacity of 300 kW, but cools150 kW. It will now be clear that this is notthe respo nsibility of th e cooling tower. It is

possible for the cooling tow er to cool 300 kWas long as this is indicated by the coolingprocess or the factorys adjustm ent. The only

correct way of measu ring the fun ctioning of the cooling tower is by m eans of the coldwater temp erature (Tw2), or the ran ge (Tw2-Tnb), as already explained in qu estion 2.

6. What is t he preferable wet bulb t emperatu re for the selection of a cooling tower?

As already ment ioned th e choice of the wetbulb temperature (Tnb) is important fordeterm ining the size of the of the coolingtower surface. For th e choice of the coolingtower w e start from a selection Tnb th at is

adjusted to th e climatological circum stancesof the environment in which the coolingtower is p laced. With this we also take th epossible recirculation of blown out air of thecooling tow er into consideration, becausethis can increase the Tnb w ith the air inlet of the cooling tower. For a correct choice of theTnb it is also important to consider the num-ber of working hours and the part of day inwhich the cooling tower is used. Duringwinter, the Tnb is as good as similar to th edry bulb temperature (Tdb), but du ring

sum mer a large d ifference between thesetwo m ay exist. With higher surroun dingtemperatures the relative humidity of air isoften lower. For examp le, an air condition of Tdb = 27 C with a corresponding R.V. = 60%accords to Tnb = 21 C.

The graph that is dep icted in figure 11 show sthe up per and lower limits of the wet bu lbtemperature in the N etherlands, measured inDe Bilt, in the period 1961- 1980 on the basisof 24 hou rs a d ay. With the h elp of this grap hwe can trace approximately how many h oursper year th e cooling tow er will not reach thedesired (selected) cooling w ater temp erature.From this it is clear that an average Tnb of 21 C is only exceeded in 0,18% of the timelooking at one year on a y early basis. In thetable below we give a short sur vey of themost common Tnb w ith the correspondingup per limits.



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For cooling towers that are situated on thecoast of the Netherlands, a wet bulb tempe-rature o f 19 C can be t aken. This has to d owith the slightly lower outside temperature

on th e coast. For climate installations in theinland of the N etherland s a Tnb of 20-21 Cis usually taken.


24

21

18

15

12

9

6

3

0100 75 50 25 0

Wett bulb temperatur ( C)

Annual percentage of exceeding (%)

figure 11. Wetbulb versus percentage of exceeding

Tnb

C

>22

>21

>20

>19

>18

Exceeding in

the Netherlands

%

0,08

0,18

0,47

1,00

1,96

Exceeding in

the Netherlands

hours/year

3

11

25

47

84


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The water consumption of an evaporativecooling tow er exists of three compon ents, i.e.evaporative losses, splash and drain waterlosses.

6.1 Evaporative lossesThe operation principle of an evaporative

cooling tower is based on the evaporation of a small par t of the circulating cooling water.The latent h eat exchang e between air andwater in an evaporative cooing tower isassumed to be 95% maximu m. The sensibleheat exchange is at least 5 %. The heatcontent of 1 kg water is 2491 kJ, so per kg.evaporat ed cooling w ater 2491 kJ energy isextracted from th e circulating cooling water.The evaporated water with correspond ingheat is absorbed in the air stream throughthe cooling tower. With the h elp of the d ata

given above, it can be d edu ced that for everykW energy that is discharged an evaporativecooling tower evaporates a max. off 1,37 Kgcooling wa ter per hou r (3600/ 2491)* 0.95= 1.37.

6.2 Splash water lossesThe splash w ater losses of an open counterflow evap orative cooling tow er are usuallydivided into splash water losses of drifteliminators (w ater that is carried by th e airstream) and losses through air inlets of thecooling tower.As a rule of thumb we can state that thewater loss of the d rift eliminators is about0,025% of the circulating cooling water debit.The splash water losses through the air inletscan be set from 0 up to 0,8% of the coolingwater d ebit. This mostly depend s on the airspeeds of the cooling tow ers location andthe qu ality of the air inlet bars. Polacel setsthe average splash w ater losses on 0,04%.

6.3 Drain water lossesSince a cooling tow er only evap orates wat er,the concentration of present salts willincrease if no fresh w ater is add ed. Theamou nt of water w ill decrease, causingundesirable chalk sediments.The amount of drain water is determined

with th e help of the so called thickeningfactor. This is the qu antity of th e salt concen-trations in th e circulating cooling w ater inrelation to the freshly added water.A minimal th ickening factor of 2 (i=2) is anacceptable starting point when the waterqu ality is good. A thickening factor of 2 tellsus that the w ater quantity that is to besluiced is equal to th e quan tity of evaporatedwater.

6.4 Example of calculationAs an examp le we use a counter flowevaporative cooling tower with:

- Circulating cooling water debit: 240 [m 3 / h]- Tw1 Tw2 (Rang e) : 10 [C]- Cooling capacity : 2791 [kW]

What amoun t of water needs to be supplied?In this example the required amou nt of supp lied w ater is 2,8 kg/ h p er kW coolingcapacity.

As a p ercentage of the circulating coolingwater d ebit this is:

- Evaporat ive losses 1,593 %- Splash water losses 0,065 %- Drain wa ter losses 1,593 %

----------- Supp lied water 3,251 %

Cooling towers 6. Water consumption 14

6. Water consumption


15/32

As a rough indication we u se figure 12. Inthis figure we can see the average requ iredamount of supplied w ater as a percentage of the circulating w ater debit for several deltaTs.

Cooling towers 6. Water consumption 15

(%)

6

5

4

3

2

1

00 5 10 15 20 Range ( C)

( i = 2 )

figure 12. Average required suppletion in function of the circulating water


16/32

7.1 Why regulating?Since the capa city of th e cooling towerincreases when the wet bulb temperaturedecreases, a regu lation is d esirable if wewan t to preserve a fixed cold w atertemperature.The cooling capacity of the cooling tow er is

preferably regulated by the amount of airthat flows through the cooling tower andthat is generated by a fan. The two sp eedregulation is a p ractical and economicallyattractive solution. By this we u nd erstandthat th e electric motor th at activates thecooling tow er fan is selected in a tw o speedmodel with a fixed revolution speed ratio of 1:2/ 3 or 1:1/ 2. An ad justable two stagethermostat regulates the fan because thethermostat changes on a fixed water outlettemp erature of the cooling tower. A good

alternative for the two sp eed electric motoris the more expensive frequency regulatedelectric motor.

7.2 Regulation of cellsWhen a cooling t ower exists of severalseparated cells, and each cell has its own fan,it seems to be useful -keeping the issue of energy saving in m ind - to switch off one ormore fans. The starting point for this is asteady cooling load on th e cooling tow er.

ExampleAs an example we take a cooling tower th atexists of three separ ate cells, which h ave acooling water d ebit of 33 m3/ h each. Thethree cooling t ower cells are placed above ashared cold w ater basin (see figure 13). Thetotal (steady) cooling load on th e three cellcooling tower is 3* 384 = 1152 kW and thedesirable cold water tem peratu re is 25 C.For several indicated wet bulb temperaturesthe cooling tower is able to cool the coolingwater from 35 C to a Tw2 that is ind icated

in table 1.The first of the three cooling cells can beswitched off when the other two can give a

cooling capa city of 1152/ 2 = 576 kW each.This is the case with a wet bulb temperatureof about 10 C and low er.In the n ext section it w ill become clear that itis not very u seful to choose a m ultiple cellcooling tower with a one speed fan / motorfor the sake of energy saving. For this, a twospeed regu lation is definitely preferred.Then, the energy use w ill be indicated, usingthe (limited) examp le below. The am oun t of mentioned Tnb hours is chosen on a yearlybasis (24 hours a day) (see table 2).If the used capacity of the three fans on h ighspeed is set on15 kW in total, the energy con-sum pt ion w ill be: 1.591* 15 = 23.865 kWh.

7.3 Two speed regulationFor a two speed regulation we assume th atthe cooling tow er exists of three air- sidedcells.Every cooling tower cell has a fan th at isdirectly driven by a tw o speed electric motorthat circles in low revolution speed on 2/ 3

of high revolution speed (think for exampleabout a 1500/ 1000 min-1 electric motor).This cooling tower is in 2/ 3 revolution

Cooling towers 7. Cooling tower regulation 16

7. Cooling tower regulation

warm

cold

figure 13. Basic arrangement three cooling towers


17/32

speed ab le to cool the cooling water from35 C to a Tw2 indicated in the table for theindicated several wet bulb temperatures.The three cell cooling tow er is, with th e 2/ 3revolution sp eed, able to give a desired coldwater temp erature of 25 C with a wet bu lbtemp erature that is lower or equ al to 16 C.For this temperature we could simultaneous-ly switch the three fans back to a lowerrevolution speed.

The theoretical energy consump tion of a 2/ 3

revolution electric motor can be calculatedwith th e help of the capacity formu la:P = (n1/ n2)3; this gives P = (2/ 3)3 = 8/ 27= 30%

Energy consumption (high revolution speed)= 53* 15 = 795 kW hEnergy consumption (low revolution speed)= 1896* 0,30* 15 = 8.532 kWhTotal 9.327 kWh

The energy saving of the two speed regula-

tion in relation to the on/ off regulation is inthis (limited) example:(23.865 9.327 =) 14.538 kWh, this is 61%.

Cooling towers 7. Cooling tower regulation 17

Tw1

Tw2

Tnb

Capacity

35

25

20

384

35

22

15

499

35

19,5

10

595

35

17

5

691

( C)

( C)

( C)

(kW)

Tw1

Tw2

Tnb

35

26,6

20

35

26,1

19

35

25,7

18

35

25,3

17

35

24,8

16

35

24,4

15

( C)

( C)

( C)

Tnb

C

20

18

16

14

12

10

8

total

Tnb

hours

25

85

232

415

533

509

499

2.298

(100%)

High

speed

%

100

88

80

74

69

64

61

High

speed

hours

25

75

186

307

368

326

304

High

speed

cells

3

3

3

3

3

2

2

Switched

off

%

0

12

20

26

31

36

39

Switched

off

hours

0

10

46

108

165

183

195

707

(31%)

Switched

off

cells

0

0

0

0

0

1

1

Tnb

C

20

18

16

14

12

10

8

total

High speed Low speed Switched off Tnb

hours

25

85

232

415

533

509

499

2.298

(100%)

%

100

33

0

0

0

0

0

hours

25

28

0

0

0

0

0

53

(2%)

cells

3

1

0

0

0

0

0

%

0

67

98

91

85

80

75

hours

0

57

227

378

453

407

374

1.896

(83%)

%

0

0

2

9

15

20

25

hours

0

0

5

37

80

102

125

349

(15%)

cells

0

2

3

3

3

3

3

cells

0

0

0

0

0

0

0

Tabel 1.

Tabel 2.

Tabel 3.

Tabel 4.


18/32

8.1 IntroductionIn order to determine the cooling towersfunctioning, we can carry out a coolingtower cap acity m easurement. For su chmeasurem ent all the entities that are relevantto the functioning of the cooling tow er arebeing measured.

We carry out th ese capacity measurem ents tocheck whether the supplied cooling towermeets its design capacity. This check isusu ally carried ou t with n ew, bigger coolingtowers, for wh ich the manu facturer and thecustomer d ecided that the guaranteed valuesare checked by means of a measurement.Another common reason to carry out acapacity measu rement is a stock-taking of the cooling tower s condition. In this case weusu ally deal with existing (older) coolingtowers that are being used for a longer

period of time. The user may have changedthe p rocess in d ue cou rse. Possibly, thenu mber of users has changed as well. Also,the fun ctioning of th e cooling tow er itself may have deteriorated. We then determinethe cond ition of the cooling tow er. In orderto d o so, we consider a correct air mov e-ment, a good water distribution, fillpollution etcetera. Often, the user is alsodiscontented w ith the cold water su pp ly. Bymeans of a capacity measurement we are notonly able to pin dow n the real functioningbut also to base conclusions or p ossibleamendm ents on these.

8.2 Measurable entitiesIn order to determine the cooling towercapacity w e measu re several entities. Theseentities together reflect the cooling tow erfunctioning. The following entities areimportant for the measuring of the coolingtower capacity (see figure 14).

Temperatures The water inlet temperature (Tw1) of the

water supply that needs to be cooled. Thistemperature is measured in th e supp ly

pipe to the cooling tower or in the waterdistribution system.

The water outlet temperature (Tw2) of thecooled w ater from the cooling tow er thatgoes to the cooling p rocess. This tempera-ture is preferably measured in the drain of the cooling tow er because in there thewater temp erature is average. The influen-

ce of the sup plied w ater needs to be fixedbecause sometimes the water supplycannot be switched off during measuring.

Cooling towers 8. Measuring the cooling tower capacity 18

8. Measuring the cooling tower capacity

ambient wet bulb

ambient dry bulb

exhaust wet bulb

exhaust dry bulb

pump head

absorbed power fan

airvolume

watervolume

wind velocity

T nb o

T nb o

T db o

T db o

T nb u

T nb u

T db u

T db u

pump

pump

ABSQL

T w1

T w2

OC

OC

OC

OC

QL

Qw

Qw

ABS

figure 14. Measurement units of a mechanical draft cooling tower


19/32

The wet bulb temperature of thesurrou nd ing air of the cooling tower. Thistemperature is measured before the airinlets of the cooling towers w ith the h elpof an Assman psychrom eter.

These temperature measurements arepreferably registered by a computer so thatpossible fluctuations in th e system can betracked down.For this, PT100-recorders are often usedbecause of their stable functioning.

Water flowIn order to fix the w ater flow (Qw) severalmethods can be used. Preferable methods arestandard measurements such as orifice platecovering (which usu ally is not very su itablefor cooling towers) or measurem ent byPitots tube. The measuring p oints, then,need to be includ ed in the d esign of theinstallation. Points to consider are a straigh tlength of pipes before and after the measu-ring point and the location of the measuring

points.The advantage of the measurement by Pitotstube is that th e user of the cooling tow er canassemble two simple ball- or pu sh cranes perpip e (a free culvert is requ ired), in w hich thePitots tube can be shoved (see figure 15).Other possible water flow measurements areelectro-magnetic or indu ctive flow method sor turbine meters, which obviously need tobe fixed as well.

Boosting of the cooling towerThe water d istribution system of the coolingtower requires a certain pu mp boosting.This boosting is defined as the su m of thepressure of the water d istribution an d thedifference in h eight between th e water inletconnection and th e bottom of the air inlet.In some types of cooling towers th e waterdistribution is without pressure because of the use of the drains and th e required pu mpboosting is the d ifference in height.

Used capacity of the fanOne of the criteria of the cooling tower func-

tioning is th e u sed electric capacity of the fan(obviously exclusively for a forced dr augh tcooling tower). For this the electric used

capacity of the driving motors is measured.If we want to know the used shaft capacityof the fan, we w ill have to includ e the effi-ciency of the d rive in th e calculation.


Installation of pitot-tube measurement connections

detail pitot-tube connection

female thread 3/4" B.S.P.

ball or butterfly

(valve min. 16 mm. access)

welded socket 3/4" B.S.P.

minimum

lenght

duct diameter 1500 mm.

mP = position of the pitot-tube connections

2/3 H = 2/3 L = min. 10x the duct dimension

RD 910

cooling tower mP (2x)(option 1)

(option 2)

mP (2x)

1/3 H

2/3 H

1/3 L 2/3 L

hole

minimum 90

16 mm.

option 1 is preferred

figure 15. Pitot-tube arrangement


20/32

Air debit and static boostingFor the inspection of the cooling tow ercapacity th e air flow is in fact of secondaryimpor tance, because th e user is only interes-ted in th e water-side temperatu res, capacityand the u sed electric capacity.In order to get a good view of the functio-ning of the cooling tower and the working of the fan, the air flow measurement usuallygives a lot of add itional information .The air debit is preferably fastened b y atraverse Pitots tube m easurement in th e fan

cone under the fan. In order to do so, fourdiagonally placed holes should be drilled inthe planes, over which the measurement canbe carried ou t. The term traverse is used fora measurement in which the surface that hasto be measured is divided into a nu mber of equal su rfaces. For each su rface the speedof air in the centre of gravity of this surfaceis measured.The result of these measurements can beaveraged quite easily. When a Pitots tub emeasurement is not possible du e to the

cooling tow er construction, we can a lsomeasure th e air debit w ith a so-calledwinged rado meter. For this we need tomeasure ov er this surface over severalpoints.In add ition, the pressure drop over thecooling tower is a useful notion when onewan ts to check wh ether the cooling tow er ispolluted . With this we can also comp are thefunctioning of the fan with th e fan curv e.

Temperature and humidity of theoutlet airThe cond ition of the air on th e outlet of thecooling tow er can be recorded with th e helpof an Assman p sychrometer.These data are especially import ant forformulating a heat balance. This heathbalance is the comparison betw een thecapacity (kW) that is released by t he w aterand the capacity (kW) that is absorbed bythe air. These capacities should be equal toone another. For the air sided heath absorp-tion the enth alpy content of the air can bedeterm ined from th e conditions of the air.

How ever, it is quite hard to measu re this,because in the outlet there are drops thatdisturb the measurement. The heat balance is

used as a coarse indication to check wh etherthe measu rement is reliable. Deviations up to20% can be prevented because the air sidedcapacity is hard to be determined more accu-rately.

Air speedThe speeds of air near the cooling tow er thatoccur d uring the measurement need to bemeasured as well. The speed of air ischecked because an air sp eed that is toohigh influences the air flow throughout the

cooling tower.

8.3 Conditions for a goodmeasurement

Obviously a cooling tower capacity measure-ment n eeds to be carried ou t un der certaincircumstan ces. Otherwise the d eviatingcircumstan ces will influence the accuracyand the reliability of the measu rement toomu ch. In ord er to carry out a capacity meas-urem ent of the cooling tower, the following

conditions must be met:- The wet bulb temperature of the surroun-ding air should not d iverge too much fromthe d esign. It shou ld p referably lie between+3K and -7K with respect to the d esign wetbulb.

- The speed of air should not be too highbecause this influences the air flow throu g-hou t the cooling tow er. Air speeds h igherthan 3 m/ sec continuously and sudd engusts of wind h igher than 5 m/ sec areconsidered to be unallowable.

- The water debit over the cooling towerneeds to be close to the d esign. The w aterdistribut ion is influenced negatively bylarge deviations of the water flow.Examples of this are bad fun ctioning of th esprinklers, insufficient m oistening wh enthe w ater flow is too low, excessive wateron the sides or flooding of the drains whenthe water flow is too high. During themeasurement, the water flow shouldpreferably lie within 10% of the designvalue. Furthermore, du ring the m easure-ment the w ater flow should be as constant

as possible. The air flow shou ld not b echanged du ring the measurement byswitching the fans. These should be set on



21/32

high throughout the m easurement.- Further deviating conditions concerning

the functioning of the cooling tow er shouldbe discussed with the user and before themeasurement agreements should be mad e.

- Obviously, the cooling tower should bechecked for its mechanical functioning.This includ es the functioning of thenozzles, pollution or da mage to the fill andthe condition of the fan and the drive.

8.4 Norms for theperformance of thecooling tower capacity

Already several norms are draw n u p for theexecution of the so-called p erformance of newly realised cooling tow ers. These norm sconsider the prerequisites under which themeasurement can be carried out, the measu-ring equipment that should be used and theworking p roceedings and evaluation techni-ques of the results.

Some well-known norms are:- DIN 1947 Thermal performance acceptan-ce testing of water cooling towers. Thisnorm is characterised by a very d etaileddescription of prerequisites, work ing-method and measuring equipment. Themeasurement is complicated and time-consuming. Measurements by this normare thu s extremely expen sive.

- CTI code ATC-105 Acceptan ce test codefor water cooling tow ers. This norm isfrequently applied in the USA and ischaracterised esp ecially by th e evaluationof measuring results with the KAV/ L-formula.

- VDMA 24419 WrmetechnischeAbnahmemessungen an zwangsbeluftetenstandarisierten Nasskhltrmen.This norm is characterised b y a simp lerapproach and method so that it is applica-ble in p ractice. A characteristic of thismethod is the calculation b ack to Tw2.

Other n orms are for examp le the BritishStandard (BS 4485-1, 4485-2, 4485-3), the

French norms according to AF-NOR (X10-251X10-252X10-253) an d the Amer ican n orm saccording to AN SI/ ASME (PTC23-1986).

At the mom ent the European cooling towermanufacturers, in co-operation withinEurovent, are draw ing up a completelyrenewed measuring p rotocol that can berealised in p ractice w ithout difficulty.

8.5 Evaluation of thefunctioning of thecooling tower inrelation to the design

In order to be able to evaluate the measuringresults in connection with the design data,the measured values need to be compared tothe cooling tow ers design d ata. There areseveral methods th at can be u sed to evaluatethe measurement. Nowadays, the followingevaluations are commonly used:Firstly, the comp arison betw een the m eas-ured KAV/ L (this is the cooling tower scode) and th e design KAV/ L. This method ,how ever, is not a p ractical evaluation for the

user s conceptualisation of the cooling tower.Another frequently used evaluationis the comparison between the design wateroutlet temperature and th e water outlettemperature that is calculated from themeasuring data and that the cooling towerreaches in practice. In fact, this methodreflects a p ractical deviation of the p resentfunctioning of the cooling tow er with respectto the design.

Obviously, calculations by comp uter enablefast calculation of the m easuring v alues of the d esign and can easily reflect deviations inthe water outlet. However, curves that showthe influence of an entity that is to bemeasured with respect to the Tw2 are stillbeing used. Examples are the two curvesdisplayed in figure 16. By m eans of thesecurves one is able, with the h elp of certaincalculation basics, to identify the p resentwater outlet tem peratu re (Tw2-ist). Thedesign data reveal what the w ater outlettemperature (Tw2-soll) should be. We canalso determine a total inaccuracy of the

measuring tolerances of the measuringinstruments and the u ncertainties of fluctuations in the system (*Tw inaccuracy).



22/32

The formu la then is:

Tw2-soll + *Tw inaccuracy Tw2-ist

In other words, the measured and recalcula-ted water outlet temperature should belower than the d esign w ater outlet tempera-ture heightened with th e total measuringinaccuracy.When th is cond ition is met, we could saythat at the moment of measuring the coolingtower complies with its design, within the

active measu ring tolerances.


Correction curve influence T nb

Correction curve influence water flow

designTw2

Tnb

25

1912 22

change

of Tw2

% Q (m /h)w

+1,0

-1,0

90% 100%

0

110%

Tw2

ambient wet bulb temperature ( C)

cold water temperature ( C)

Tnb

water flow (m /h)

cold water temperature ( C)

Q wT w2

3

3

figure 16. Examples of correction curves for Tw2


23/32

9.1 IntroductionA cooling tower exists of a num ber of fun c-tioning components that can be found inevery cooling tower in a certain form. As anexample we consider a mechanical draughtcounter flow cooling tower. The m ain com-pon ents of this cooling tower are:

1.The casing of the cooling tower. The casingis built from e.g. wood , steel or synth eticfabric. The casing may be self-carrying or itmay be moun ted on a metal frame.

2.The air inlet bars. These should take carethat the minimal amount of water spu rtsthrough the air inlets and that the air is ledinto the cooling tow er in an op timal way; i.e.the air should not choose the shortest way tothe fan. Therefore, the bars are u sually d irec-

ted d ownw ards, so that the air is led furtherinto the cooling tower and streams into thecooling fill more gradually. These bars can bemad e of synth etic material, steel or wood .

3.The cooling fill. The different types of cooling fills and their functioning are alreadydiscussed extensively in p art 2 of these seriesand therefore will not be furth er discussedhere.

4.The water distribution system will bediscussed in more detail in this part.

5.The drift eliminator w ill be discussed inthis part.

6.The fan section. For an ind uced d raftcooling tower (see figure 18) this consists of an induced draft fan, driven by an electro-motor. For bigger cooling towers th e revolu-tion speed of the electro motor n eeds to bereduced by m eans of a reduction component.This because of the maximum tip speedsthat, for e.g. synthetic fans lie between 50

and 70 m/ s., depending on th e constructionand the choice of materials. This reductioncomponent can exist of a geared motor that

dr ives the fan directly or of a right anglegearbox with an intermediate shaft and a

foot moun ted m otor, wh ere this electro-motor usu ally is outside of the fan stack (seefigure 18).

Cooling towers 9. Cooling tower components 23

1. kast

2. air intake louvres

3. fill

4. water distributor

5. drift eliminator

6. fan section

motor

direct driven fan

geared motor

indirect driven by a geared

motor

motorgear box

Indirect driven by a rightangle gear box

9. Cooling tower components

figure 17. Functional cooling tower parts

figure 18. Impeller arrangement induced draft


24/32

Obviously, a good technical adjustmen tof these components and the optimalfunctioning of the cooling tow er deserveextra attention.The following p oints are taken intoconsideration:- The relative proportion of the fans

diameter compared to the drift elimina-tors surface and the cooling fills surface.We choose a large enou gh fan, so that noextreme air speeds occur in the fan conewh ile there is a correct air speed in the

cooling fill and th e d rift eliminator(the air speeds in th e fan cone vary from1-15 m/ sec).

- The distance between the fan and thedrift eliminator should be large enough toprevent local differences in air speed in th edrift eliminator.

- The inlet shape of the fan cone and thefans tip spa ce in th e cone are importan tto the ultimate fan p rofits and thereforefor the energy consump tion of the coolingtower as w ell.

- The nozzles should be distributed in suchway that the water streams through th ecooling fill instead of along th e sides of thecasing, for water streaming along th e sideswill not be in sufficient contact with thedraw n-in air. This portion o f water willthen redu ce the cooling capacity.



25/32

9.2 The drift eliminatorsection

Principle of operationThe functioning of a d rift eliminator is basedon th e inertia of a drop of water. Every drifteliminator incorporates a specially construc-ted curve or tu rning brim that causes the airto chang e d irection. Because of this changeof direction, the d rop flies out of the curveand is caugh t in the d rift eliminator s profile.From this pr inciple follows that for the d rift

eliminators th ere is a minimal speed forwh ich th e drop s still fly out of the curve.Also, a drift eliminator is best in catching thedrops when the speed is high (the flow-inspeeds for a d rift eliminator lie between 2 to4 m/ sec). Below the minimal speed th esmaller drop s can wh irl throu gh th e drifteliminator.Of course th ere also is a maximum sp eed forthe d rift eliminator; this has to do withturbulence and too much resistance.

Drift eliminators for cooling towers areusually produced in synthetic materials likePVC and PP. The reason for this is that thesematerials are more aerodynam ic in designthan e.g. wood. For special uses in th eprocess industry, where high temperaturescan be reached , they can also be man ufactu-red in stainless steel.When placing the drift eliminators we makesure th at the ad jacent eliminators are fittedclose together because if slits occur, dropscould fall through th em.

Horizontal drift eliminatorThis type of drift eliminator is used incounter flow cooling towers (see figure 17).The air speed in this dr ift eliminator is about2-4 m/ sec. In th ese drift eliminators thesmaller wh irling drop s are assembled intobigger drops which then fall back into thecooling tower (see figure 19).

Vertical drift eliminatorIn this type of drift eliminator t he drop s arecollected in sp ecial drains th rough wh ich

they are drained aw ay in down ward d irec-tion. When constructing this typ e of drifteliminator w e take care that the w ater can be

carried back from th e bottom of the d rifteliminator to the basin. The air speed of thistype of drift eliminator may be between 2and 8 m/ sec, depend ent on the design andconstruction.


drift eliminators

a i r f l o w

X

nodrops

drops

plastic profiles wood

dimension X between 150 - 200 mm.

intermediate distance between 25 - 40 mm.

air flow

dimension X between 150 - 200 mm.

intermediate distance between 25 - 40 mm.

X

drift eliminators

water drain

air flow

nodrops

drops

figure 19. Basic principle horizontal drift eliminators

figure 20. Basic principle vertical drift eliminators


26/32

9.3 The water distributionsystem

For the distribution of w ater in a coolingtower several systems can be used.A condition for a well-functioning waterdistribution system is that the water is equal-ly distributed over the cooling fill in order toobtain optimal contact between water andair.

Cross and counter flow cooling

towersIn counter flow cooling t owers (see figure18) water distribution systems that dispersethe w ater in a fine drizzle over the fill arealways u sed. Two system s are recognised:

Regular low pressure nozzlesThese nozzles are usually carried ou t accor-ding to the full cone pr inciple, in order th atthe entire surface beneath the n ozzle can bewetted . The boosting u sually lies between20 and 50 kPa. The water is distributed by

means of a so-called wh irl-plate that causesa good spraying result. When the p ressureover the nozzle drops too low, the sprayingresult decreases and the spraying angle willbecome smaller than that it was d esigned for.When th e pressure over the n ozzle is toohigh, the sh ape of the full cone nozzlechanges to a hollow cone nozzle because thewater is hurled ou t more. The spraying angleusu ally is 120; therefore the build ing h eightof the part of the nozzles can remain low.

Splash nozzles without pressureThese nozzles operate by the p rinciple of gravity, wh ich causes the w ater to streamthrough open drains or half-filled pipes tothe spraying points.The drawback of open drains is that there isthe chance of heavy algae growth , which cancause blockages. Through an outlet pipe thewater falls on a sp ecially constru cted d isc,wh ere it diffuses into a hollow sprayin gresult. The ad vantage of this n ozzle is thatpressure is not required. The disadvantage isthat th ese nozzles do n ot form a full cone

because of the d isc construction. For thisreason the fill right beneath th e nozzle does

not get w et. In ord er to w et every fill, thenozzles must ov erlap one anoth er. In general,the water distribution with splash nozzles isnot as good as with low pressure nozzles.How ever, for the bigger cooling tow ers withsomewh at coarser fill types the sp lashnozzles suffice.

9.4 Water distributionsystem for cross flowcooling towers

In cross flow cooling tow ers a w aterdistribution system w ithout p ressure is mostcommonly used. In this system the water isdistributed over the fill by a distributionbasin with a sp ecial perforated p attern (seefigure 21). These perforations a re u suallycarried out in the form o f so-called top s,which are tubes with a fixed outer diameterand varying inner diameters. This enables usto adjust the d iameters of the hole in thebasin to a design for a d ifferent w ater debit.The large number of tops per m3 alreadydeals with the water distribution quite well.By u sing a redistribution fill between th e

water distribution system and the cooling fillan optimal water distribution is realised.


water supply duct water distribution basin with holepattern

re-distribution fill

fill

figure 21. Water distribution cross flow cooling towers


27/32

A cooling tow er is always d irectly connectedto the op en air because the tower cools usingsurrou nd ing air. Obviously, completely built-in constructions can be thought of that makeuse of sucking and dr aining chan nels.This, how ever, is not a norm al situation.Moreover, the cooling tow ers are usu ally

located on a high level on the roofs of buil-dings or on wasted p laces on the terrain thatmay be close to a border.The modern forced draught cooling tower isprovided w ith a fan with drive, which is apotential sound source. Another source of sound may be the water falling in the cool-ing towers basin. This sound may beannoy ing because of its high p itch.In the present nuisance acts and environ-mental laws increasingly higher demandsare set up for cooling tow ers. In th is chap ter

we w ill elaborate on the sou nd-produ ction of cooling tow ers. We only consider elementaryknowledge of a sound-source with a conti-nuu m devoid of peeks.

What is sound?Sound is a vibration of air that is causedby mov ement of an object. This movem entcauses differences in p ressure that m ovethrough the air in the form of vibrations.Human beings sense these vibrations bymeans of sound .

Hu mans can sense sound pressures thatrange from 20 Pa (audibility threshold) to200 Pa (pain threshold). The force of thesound pressure is defined in decibels in thefollowing manner:

P = occurring sound p ressureP o = sound pressure of the audibility thres-hold (20 uPa)

The value of the sound pressure alwaysneeds to be given together with the distanceto the source. In our formula the soun d pres-

sure of the aud ibility threshold will be 0 dBand of the pain th reshold this w ill be 140 dB.

Besides the strength (the loud ness that canbe heard), sound can also be heard in d iffe-rent p itches. These p itches occur because of the speed of chan ges in pressure; this is

called frequency. For very rap id chan ges, ahigh frequency, we hear high pitches. Forslow changes we h ear low pitches; the bastones. In general, the human ear can hearsound frequencies of 16-20.000 Hz.

A third well-known entity is the soundcapacity. This sound capacity is d efined ana-logous to the sou nd pressure. The differenceis that for the sound capacity the startingpoint is not a p ressure, but a capacity (Watt)that is broug ht into th e air.

Lw = Current sound capacityw0 = Reference sound capacity (10-12 Watt)

This value is independent of the distanceand cannot be measured d irectly with themeasuring equipment. The sound capacity isfrequently u sed to characterise the force of aspecific sound source.We can calculate the soun d capacity and thesoun d pressure (on a sp ecific distance) withthe help of for example the formu la mentio-ned below. This formu la only accoun ts for apoint source in an acoustically free fieldwithou t influence from the soil or reflections:

Lw = L p+10log(4* *R 2)R = distance to the source, the radiu s (m).

To work with these entitiesIn pr actice, the entities mentioned aboveare pu t to u se in different way s. In fact, wecan determine the sound pressure for all

frequencies with a sp ectrum-analyser.How ever, this is so extensive that in p racticefor the sake of simplicity as well as for

Cooling towers 10. Sound of the cooling tower 27

10. Sound of the cooling tower

PP

Lp = 20*log (d B)

WW

Lw = 10*log (d B)


28/32

comparison- a num ber of midd le frequenciesare used, for which the total spectrum isdivided into ranges that are shown in thetable below.Add itionally, in order to reflect the sou ndwith a simple num ber, a filter spectrum thatis in accordance with the h um an au dibilitycan be used . For this spectrum , it is resear-ched w hich p itches are the most ann oyingfor human beings. The commonly useddB(A) filter is also shown in the ta ble below:

From the A-spectrum can be conclud ed th athigh p itches especially are experienced as

annoying.In order to determine the va lue from th edB(A) spectrum, the middle frequency valu-es are added logarithmically according to theformula below:

In practice, the sound m eter is provided witha d B(A) filter, so that the d B(A) sound pres-sure level can be read directly.

Rules of thumb for calculationswith soundFor calculations with sound there are a num -ber of rules of thumb that can be app lied

quite easily. For several sources count s that

the sound pressures of the sources in ameasuring p oint can be add ed logarithmical-ly using the following formula:

Lptotal = 10 log[10(Lp

source 1)+10

(Lpsource 2

)+10

(Lpsource 3

)...]

10 10 10

For several equal sources for the sound pres-sure increase in the examp le coun ts the follo-wing:

-For 2 equal sources = 10 log(2) = +3 dB

-For 3 equal sources = 10 log(3) = +5 dB-For 4 equal sources = 10 log(4) = +6 dB

When the d istance from m easuring pointto sou rce changes, coun ts theoretically,preserving air- and grou nd d amp ening, aresonance of other in fluen ces:

For rotating m achines it counts that w henthe revolutions are changed, it can be stated

(theoretically) that LPnew can become LPoldfollowing the following formula:

An example for electrically driven machinesis:

- 2/ 3 rotation s = +50log(0.67)= -9 d B- 1/ 2 rotation s = +50log(0.5) = -15 dB

We must note here that for fans moreinfluences are of importance, which we willexamine in due course of this booklet.

The sound of the fan and thedriveThe fan is a possible soun d sou rce. Thissource is examined thoroughly, in the courseof which it is tried to grasp the soun d p ro-du ction in an empirical formula:


(value - correction valu e)

10dB(A) = 10log[10 +


10+10 +


10+10 +

+........................)

distance newdistance oldL = L -20log ( )Pnew Pold

original speednew speedL = L -50log ( )Pnew Pold

L = L + 10logW + 10logWA WASUD

3

Medium frequency Band l imi ts dB(A) fi lt er

63125250500

1000200040008000

-26-16

-9-3011

-1

45 - 9090 - 180

180 - 355355 - 710

710 - 14001400 - 28002800 - 56005600 - 11200


29/32

Lw a = the soun d cap acity of the fan d B(A)Lw as = the specific sound capacity of theused fan dB(A)W = the fan capacity kWU = rotation speedD = the fan d iameter m

From the above we can conclude, concerningan op timal fan-sound, th e following: The rotation speed of the fan is the most

influential factor. This rotation sp eed isequal to the rotation. A fan having the

same amou nt of air and boosting w ith alower rotation speed will thereforeprodu ce less sound .

The diameter also influences the soundprodu ction of the fan. A fan with a largediameter w ith the same d esign conditionswill bring abou t less sound .

The used capacity of the fan is also aninfluence on the sound. Because thecapacity is the same as the (air debit)boosting divided by the profit, startingfrom a given required debit and boosting,

only th e profit is relevant. Therefore, a fanwith a higher profit will produce lesssound.

When the above technical design aspects arethoroughly examined but too much sound isprodu ced anyw ay, we can check the outletdam pers on the cooling tower (see figure 21).The disadvantage of an outlet damper is thatit causes add itional air- sided resistance,wh ich increases the fan cap acity. Theprodu ction of the sound source (the fan)then increases according to the formulaabove. It may now be clear that from a tech-nical and economical viewpoint maximisingthe fan drive by a relatively more expensivefan and one tha t is as large as possible (witha maximised blade shape an d m ore profit forlower rotation speeds) is preferred.Good red uced-sound fans are available thesedays, so that now the drive of the fan plays apart. For a cooling tow er drive we payattention t o the electro-motor for electricsound and cooling fan sound. When, on topof that, a rotation-reduction by a gearbox or

by a redu ctor is being used, we also payattention to soun d of the gear w heel. Whenthis happ ens, a quickly rotating intermediate

stage can cause problems. When th e soundof the d rive is significantly less than the fansound in practice, a minimal difference of 10 dB is stated a nd the d rive will, as a ru le,

not cause p roblems.It is ad visable to comp are this for the entirespectrum, so that p ossible peak sound s inspecific frequencies are not an noying.

The sound of falling waterA counter flow cooling tower prod uces notonly fan sound but also splash sound of thewa ter tha t falls from the cooling fill into thebasin. A counter flow cooling tower isexplicitly men tioned , because in a cross flowcooling tower the water streams directlyfrom the fill into th e basin, an activity w hichdoes not prod uce sound an d is therefore oneof the ad vant ages of a cross flow coolingtower. For the modern smaller counter flowcooling to wer with a film fill (e.g. the PolacelCMC and CMD type) it counts that ingeneral the sound pressure measured in theair inlet bar is about 81-84 dB(A).The sound produ ction is, however, verymu ch depend ent of construction m atters as: Falling height in the cooling tower; Water load over th e cooling fill; Type of cooling fill and t he size of the

drops; Construction of the basin and the possible

water level.


msec

discharge silencer

distance1.00 m 2.00 m 3.00 m 4.00 m

noise source (ventilator)

noise source (falling water)

figure 21. Cooling tower with discharge silencer


30/32

Every manufacturer of cooling towers willgive a personally measured sound level thatis determined by means of equal coolingtower types. As an example of a sound p res-sure level that can happen in practice, thefollowing sound pressures are given for aCMC-type cooling tower of about 2.5 *2.5meter:

In order to reduce the sound of the fallingwater, we can p lace side-wing d amp ersaround the cooling tower or w e can place a

sound wall at some distance of the coolingtower (see figure 22).It is also possible to make soun d facilitiesin the basin you rself. Slanting fences orespecially constructed , floating ma ts thatcatch the drops and break them, causing lesssplash sound, could be p laced in the basin.These damp ers can redu ce the sound abou t10 to 11 dB(A).

Example of a sound-situationAs an example w e choose a standard coolingtower of 2.5 * 2.5, with a height of 3.5 metersand with a stand ard fan. This fan shouldmov e 19 m3 / sec air with a boosting of 140Pa. The used capacity of the fan is then 4.8kW with a rotation of 720 rpm.

For this fan coun ts on 10 meters a soundpressure release of:

If we w ant to rep lace this type of fan by a

reduced-sound typ e that can d o with lessrevolutions, we can realise less soun d relea-se. For a u sed capa city of 4.9 Kw and a revo-

lution of 501 rpm this fan su ffices. For thisfan counts a sound pressure release on 10meters of:

For the falling w ater of this cooling tow ercounts, as already m entioned in the formersection, a sound pressure release on 10meters of:

When w e consider the total sound p ressurelevel of this cooling tow er w e will find as asum of the water sound and the fan soundfor the standard situation on 10 meters asound pressure release of:

In the situation w ith a reduced-sound fancounts on 10 meters a sound pressure releaseof:


Medium frequency Air intake Sound pressure levelat 10 m.

63

125250500

1000200040008000

dB(A)

50

5048535655565461

72

7270757877787683

canting baffleplates floating

silencers

25 - 30

A: canting baffle plates B: floating noise attenuators

watersurface

figure 22. Attenuation of the noise by the falling water

Hz

dB

63

61

125

63

250

64

500

62

1000

59

2000

54

4000

48

8000

42

dB(A)

64

Hz

dB

63

53

125

56

250

58

500

56

1000

53

2000

49

4000

42

8000

35

dB(A)

58

Hz

dB

63

50

125

50

250

48

500

53

1000

56

2000

55

4000

56

8000

54

dB(A)

62

Hz

dB

63

55

125

57

250

58

500

58

1000

58

2000

56

4000

56

8000

54

dB(A)

63,5

Hz

dB

63

61

125

63

250

64

500

62

1000

61

2000

57

4000

56

8000

54

dB(A)

66


31/32

Considering the above, we see that especial-ly in the reduced-sound situation the w atersound plays a great part. In this situation wecould with the help of for example floatingdam pers (sound attenuators) reduce thesound of falling wa ter to a soun d releasefrom 10 meters of:

For the total sound of the combination

reduced-sound fan w ith d ampened watersound, counts on 10 meters:

ConclusionWe find that by the use of a sound-reducedfan and by dam pening of the splash watersound a sound reduction from 66 dB(A) to59 dB(A) of the standard fan and the sound

of the w ater is possible at a d istance of 10meters. Obv iously, this situation is just a nexample. Every situation should be conside-red sep arately.


Hz

dB

63

50

125

50

250

48

500

48

1000

44

2000

45

4000

45

8000

39

dB(A)

52

Hz

dB

63

55

125

57

250

58

500

57

1000

53

2000

50

4000

47

8000

40

dB(A)

59


32/32

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